A novel form of the G protein beta subunit Gbeta5 is specifically expressed in the vertebrate retina.

The G protein beta subunit, Gbeta5, is predominantly expressed in the central nervous system. In rodent brain, Gbeta5 is expressed as a protein with an apparent molecular mass of 39,000 daltons (39 kDa). We have identified an additional Gbeta5 immunoreactive protein of apparent size 44 kDa in the vertebrate retina. Molecular cloning and sequencing of polymerase chain reaction products revealed that the cDNA encoding the larger species of Gbeta5 (Gbeta5L) was identical to the shorter form with the addition of 126 base pairs of 5' DNA sequence potentially encoding an in-frame 42-amino acid extension. Sequencing of mouse Gbeta5 genomic clones demonstrated that the 126-base pair of retinal-specific coding material is derived from a hitherto undetected 5' exon. During sucrose density gradient fractionation of bovine retinas, the 44-kDa Gbeta5L protein co-purified with rod outer segment membranes. Incubation of rod outer segment membranes with the nonhydrolyzable guanine nucleotide, GTPgammaS (guanosine 5'-3-O-(thio)triphosphate), which released the Gbeta subunit of transducin (Gbeta1), failed to remove Gbeta5L. The 39-kDa Gbeta5 protein displayed differential association with retinal and brain membranes. In the retina, Gbeta5 was present as a soluble protein and was undetectable in the membrane fraction, whereas in the brain approximately 70% of Gbeta5 was associated with cellular membranes. In transient COS-7 cell expression experiments, Gbeta5L formed functional Gbetagamma dimers and Galphabetagamma heterotrimers, and activated phosphoinositide-specific phospholipase Cbeta2 in a manner indistinguishable from the 39-kDa Gbeta5 protein. The cloning of the retinal-specific Gbeta5L cDNA suggests the existence of potentially novel G protein-mediated signaling cascades in photoreception.